The invention relates to a process for controlling coloration in multicolor printing.
Prior art solutions use an image recording arrangement for continuous determination at a plurality of measuring locations of actual color measurement values which are then compared with setpoint color measurement values in a controlling device. The signals resulting from this comparison are supplied to color controlling elements which adjust the layer thickness of the colors to be printed on top of each other such as to reduce the difference between the actual color measurement values and the setpoint color measurement values. To achieve high accuracy, the largest possible number of actual color measurement values is obtained with each printing, if at all possible.
The increase in the number of measuring locations is limited by the measuring geometry of the image recording elements and by the finite processing speed of the hardware components used for controlling. Processes have therefore been proposed wherein, for example, measured values are combined by ranges, or only a portion of the measured values is used, or the measured values from less than each printing are used.
The object of the invention is to define a process for controlling coloration which makes it possible to reduce the time required to obtain a desired coloration while maintaining high accuracy.
This object is attained by a process having the characteristic features defined in claim 1.
According to the invention, the actual color measurement values are determined in a first step at only selected measuring locations. In practice, this means that approximately 500 actual color measurement values are determined, for example, in a sheet-fed printing machine producing, for example, prints of a 1020 mm.times.750 mm format. A sensor for one actual color measurement value covers a range of approximately 3 mm.times.3 mm such that the 500 measuring locations cover less than 1% of the area of the sheet. The measuring locations selected in the first step are image-relevant positions which are particularly significant for coloration. As a rule, these are measuring locations in gray tones of a printed image in which color deviations are particularly readily observable by the human eye. The low number of measuring locations makes it possible to determine the actual color measurement values of each sheet.
In a second step, the actual color measurement values obtained from the selected measuring locations are compared with setpoint color measurement values. The resulting monitoring values are checked to determine if they exceed a given threshold value. If this is the case, actual color measurement values are then obtained once from complete image ranges up to the entire printed image. Said image ranges cover, for example, more than 10% of the printed area, whereby non-printed areas can be excluded for the purpose of measured value processing. From this substantially increased number of actual color measurement values, controlled variables are derived by comparison with set point color measurement values and are supplied to a color controlling device. Controlling elements of the color controlling devices influence the color on the printed material, for example, by changing the layer thickness of the colors to be printed on top of each other, in wet offset printing, by changing the proportion of wetting agent in an emulsion of ink and wetting agent, or by actuating register control devices or devices for changing the hue value.
The process can be used both at startup of the printing process and during a continuous printing operation. While the controlled variables are determined, which takes a certain amount of time due to the large quantity of data from the plurality of measuring locations, actual color measurement values from the few selected measuring locations are continuously processed into monitoring signals. Processing of actual color measurement values from the complete image ranges or from the entire printed range is required only at those instants when an invalid state of coloration is detected during processing of actual color measurement values originating from the selected measuring locations. To minimize errors in color control, one variant of the invention provides for averaging the actual color measurement values from the complete image ranges or the entire printed range of several sheets. Averaging of the actual color measurement values of several sheets can be carried out by means of a device based on hardware components or by means of a computer that comprises a corresponding program.